This invention relates to a display panel having an organic polymer film.
In conventional dispaly panels, a display member such as a liquid crystal or a stron dielectric is interposed between a pair of glass substrates. To reduce the size and weight of the display panels, however, the use of a organic polymer film as the substrate has been proposed in recent years.
FIG. 1 illustrates the structure of a conventional reflection type liquid crystal display panel.
FIG. 1(a) is a plan view and FIG. 1(b) is a sectional view taken along line A--A' of FIG. 1(a).
In the drawings, reference numerals 1a and 1b represent the organic polymer film substrates; 2a and 2b are electrodes of transparent conductive layers consisting of a mixture of 95 wt % In.sub.2 O.sub.3 and 5 wt % SnO.sub.2 that are disposed on the opposed surfaces of the substrates 1a, 1b; 3 is an electrode terminal for connecting the electrodes 2a, 2b to an external circuit; 4 is an orientation film; 5 is an epoxy-type adhesive used for bonding the periphery of the substrate 1a, 1b; 6 is a liquid crystal sealed between the substrates 1a, 1b; and 7 is a reflecting plate. Though not shown in the drawings, polarizers are disposed on the non-opposing surfaces of the substrates 1a, 1b, whenever necessary. Reference numeral 8 represents incident light. Symbol d represents the deviation between a virtual image 9 and a real image 10 while symbol .phi. represents a viewing angle.
To use the organic polymer film as the substrate for the display panel, the following requirements must be satisfied:
(1) The film must be highly heat-resistant.
(2) It must also be highly resistant to acid and alkali.
(3) It must have high mechanical strength. If so, gap control can be easily made between the substrates and the thickness of the substrate can be reduced, thus making it possible to reduce also the size and weight of the display panel. The deviation d between the virtual image 9 and the real image 10 can be reduced by reducing the thickness of the substrate in the case of the reflection type display panel.
(4) It must have high light transmittance.
(5) It must have high viewing angle dependence of the display quality. In other words, its light transmittance must not drop even if the viewing angle .phi. becomes great.
(6) The transparent conductive layer must not be viewed from outside. The reflectivity R of the conductive layer is expressed by the following equation: EQU R=[(n.sub.e -n.sub.s)/(n.sub.e +n.sub.s)] (1)
where
n.sub.e : refractive index of the transparent conductive layer 2 (.apprxeq.2.0) PA1 n.sub.s : maximum refractive index of the substrate 1.
The smaller this reflectivity R, the smaller becomes the possibility that the transparent conductive layer can be seen from outside, and the higher becomes the display quality.
The maximum refractive index n.sub.s of the substrate will now be explained. A stretched organic polymer film literally means a organic polymer film produced by the stretching operation. After passing through the stretching operation, the stretched organic polymer film has by far higher transparency and mechanical strength than an unstretched organic polymer film. However, stretching of the film results in the occurrence of molecular orientation and the film becomes optically anisotropic. FIG. 2 is useful for explaining this phenomenon. FIG. 2(a) depicts a biaxially stretched film that is stretched in two directions, i.e, A--A' and B--B', while FIG. 2(b) depicts a uniaxially stretched film that is stretched only in one direction, that is, A--A'. In either case, three kinds of slow and fast principal axes of birefringence a, b and c exist and have refractive indices n.sub.a, n.sub.b and n.sub.c, respectively. The maximum refractive index n.sub.s is defined as the greatest of these three refractive indices.
(7) The maximum refractive index n.sub.s of the substrate must be great. In other words, the deviation d between the virtual image 9 and the real image 10 can be reduced in case the display panel is of the reflection type.
(8) If the display member consists of a liquid crytal, the film must have high resistance to dissolution in the liquid crystal.
(9)If the display member consists of the liquid crystal, the film must have high solvent resistance because an organic solvent is used in forming the orientation film 4.
As the organic polymer films for use as the substrates of the display panel, acryl resin films produced by uniaxial stretching methods, tri-acetylcellulose films produced by uniaxial stretching methods, polystyrene films produced by uniaxial stretching methods (see: Japanese Patent Laid-Open No. 97318/1981), and polyester films produced by biaxial stretching methods have been known conventionally.
As will be described elsewhere, however, none of these prior art films can entirely satisfy the requirements (1) through (6), especially the requirements (1) through (7) if the display panel is of the reflection type and the requirements (1) through (6) and the requirements (8) and (9) if the display member consists of the liquid crystal.